WO2017192196A2 - Laser à fibre à impulsions ultracourtes employant la diffusion raman dans des fibres de mode d'ordre supérieur - Google Patents

Laser à fibre à impulsions ultracourtes employant la diffusion raman dans des fibres de mode d'ordre supérieur Download PDF

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WO2017192196A2
WO2017192196A2 PCT/US2017/017645 US2017017645W WO2017192196A2 WO 2017192196 A2 WO2017192196 A2 WO 2017192196A2 US 2017017645 W US2017017645 W US 2017017645W WO 2017192196 A2 WO2017192196 A2 WO 2017192196A2
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fiber
wavelength
mode
fiber laser
hom
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WO2017192196A3 (fr
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Siddharth Ramachandran
Lars RISHOJ
Jeffrey D. DEMAS
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Trustees Of Boston University
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
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    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
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    • H01S3/16—Solid materials
    • H01S3/17—Solid materials amorphous, e.g. glass
    • H01S3/171—Solid materials amorphous, e.g. glass chalcogenide glass

Definitions

  • the present invention is related to the field of fiber lasers.
  • ultrafast, high-energy, high repetition-rate laser pulses in a variety of wavelength ranges is of interest in multiple applications.
  • multiphoton microscopy commonly used for deep tissue brain imaging, requires sources at wavelengths where fluorophores are available. Desirable operation is achieved using an excitation source having a wavelength in either of two tissue transparency windows at approximately 1300 and 1700 nm.
  • ultrafast, energetic near-infrared or mid-infrared sources may be required or desirable. These applications may require peak pulse powers at megawatt (MW) levels.
  • high repetition-rate may signify sources that emit pulses at the rates of 100 kHz or more.
  • a fiber laser may be realized using a gain medium that emits photons at a desired color (wavelength).
  • Most common gain media that can be incorporated in optical fibers are realized using rare earth dopants, such as Ytterbium (in the 1000 nm range), Erbium (in the 1550 nm region), or Thulium (in the 2000 nm range).
  • Fiber-based lasers offer tremendous opportunities and have been successfully deployed in a variety of applications.
  • high energy ultrashort pulse fiber lasers Due to a lack of laser gain media at other wavelengths, high energy ultrashort pulse fiber lasers have generally been restricted to operation in the above three specific wavelength regions (1000 nm, 1550 nm, and 2000 nm).
  • wavelengths in the near-IR generally about 700 to 2000 nm
  • mid-IR generally greater than about 2000 nm
  • Specific ranges of interest may include 700-1000 nm, -1300 nm (e.g., 1200-1400 nm), -1700 nm (e.g., 1600-1800 nm), and even longer wavelengths such as 2-10 ⁇ .
  • a fiber laser that includes a higher-order-mode (HOM) fiber, having a step index and a guidance diameter, that supports multiple modes of optical signal propagation, each mode has a respective wavelength-dependent dispersion
  • HOM higher-order-mode
  • the higher-order modes include a predetermined higher-order mode with corresponding anomalous dispersion characteristic and effective area, and a first wavelength and first power of a pulse optical signal is converted to a second wavelength and second power by soliton self-frequency shifting (SSFS) in the HOM fiber.
  • the second wavelength and second power are related to the first wavelength and first power by the anomalous dispersion characteristic and effective area for the predetermined higher-order mode.
  • the fiber laser further includes a source subsystem coupled to the HOM fiber to establish the pulse optical signal propagating in the HOM fiber in the predetermined higher-order mode.
  • the second wavelength may be shifted into a desired range even while the first wavelength is away from that range, and thus the laser may be used in applications requiring non-conventional fiber laser wavelengths.
  • a fiber laser in another aspect, includes a higher-order-mode (HOM) fiber, having a step index and a guidance diameter, that supports multiple modes of optical signal propagation, each mode has a respective wavelength-dependent dispersion, group index, and effective area characteristics.
  • the higher-order modes include predetermined first and second modes sharing a predetermined group index (or group velocity) at corresponding first and second wavelengths to define a first pulse optical signal having the first mode and first wavelength from which a second pulse optical signal of the second mode and second wavelength is produced by interpulse and intermodal Raman scattering.
  • a source subsystem is coupled to the HOM fiber to establish the first optical signal propagating in the first mode to produce the second optical signal.
  • the predetermined wavelength difference between the first and second wavelengths are similar or close to the peak of the Raman gain coefficient with respect to the first wavelength.
  • the conversion process may have a cascaded aspect in which each converted signal serves as a pump or initiator for a next converted signal having a next separated wavelength, and thus a greater range of tunability may be realized compared to SSFS.
  • Figure 1 is a block diagram of a fiber laser system
  • Figure 2 is a schematic diagram of a refractive index profile of a HOM fiber supporting LP 0; m modes;
  • Figure 3 shows an alternative fiber with a double cladding structure and multiple guidance regions
  • Figures 4A and 4B illustrate another fiber having a guidance region diameter of 84 ⁇ and refractive index profile as shown;
  • FIGS 5 A and 5B illustrate an example HOM fiber in which the guidance region is defined by low index polymer
  • Figure 6 illustrates an alternative in which the guidance region is defined by a ring of airholes
  • Figure 7 shows index profile for a fiber using non-silica material, for mid-IR wavelength operation
  • Figures 8 A and 8B show an example of a fiber that can be used for guiding OAM modes
  • Figure 9A shows plots of dispersion and effective area (respectively) versus wavelengths for a number of LP 0; m modes;
  • Figure 9B shows an example of the spatial intensity distribution of the LP 0 9 mode
  • Figures 10A and 10B shows dispersion curves for a single mode for different combinations of guidance diameter and index step;
  • Figures 11 A and 1 IB illustrate dispersion versus wavelength and effective area versus wavelength for selected modes in a fiber having a guidance region of 84 ⁇ ;
  • Figure 12 is a plot of dispersion versus mode order at a wavelength of 1050 nm for two different fiber types;
  • Figure 13 illustrates dispersion curves for a fiber such as that of Figure 6 in which a large index step is obtained by surrounding the guidance region with a ring of low index air holes;
  • Figures 14A and 14B show dispersion and effective area for the fiber of Figure 7;
  • Figure 15 presents a simulation of the dispersive characteristics of OAM modes versus wavelength for a fiber such as that of Figure 8;
  • Figure 17 shows a system arrangement
  • Figure 18A shows a setup that exhibits SSFS in LP 0; m mode;
  • Figure 18B is a plot of dispersion and effective area versus wavelength for the LP 09 - mode excited using the setup of Figure 18 A;
  • Figures 19A - 19C are plots of output spectra, autocorrelation measurements, and output mode re-conversion
  • Figure 20 is a plot of power versus wavelength illustrating SSFS in a low-index polymer coated fiber
  • Figures 21 A and 2 IB show soliton shifting spectra and mode image respectively in an air-cladding fiber
  • Figure 22 is a plot of power versus wavelength showing soliton shifting in an OAM mode in an air core fiber
  • Figure 23 is a plot of output energy versus wavelength for fibers of different lengths
  • Figure 24 is a plot of a dispersion profile for the LP 0; 9 mode of a HOM fiber;
  • Figure 25 is a plot showing that soliton spectral bandwidth increases with increasing soliton shift in a mode with dispersion characteristics as shown in Figure 24;
  • Figure 26 is a plot of Raman gain coefficient for silica as a function of wavelength
  • Figure 27 is a plot of group index versus wavelength for a selection of modes in a fiber
  • Figure 28 is a plot of effective Raman area versus cascade order
  • Figure 29 is a plot showing a comparison between the strength of different intermodal processes
  • Figures 30 and 31 are plots similar to those of Figures 28 and 29 for a different fiber
  • Figure 32 shows an output spectrum for an experiment in which the pump pulse is launched in the LP 0; i8 mode in a fiber with a guidance region of 105 ⁇ ;
  • Figure 33 illustrates a simulation showing that different modes at different wavelengths all have the same group index;
  • Figure 34 shows an experiment in which the LP 0; 8 mode is launched in a fiber with a guidance diameter of about 50 ⁇ ;
  • Figures 35 A - 35E show mode images of spectral features
  • Figure 36 presents plots of simulated group index showing that at least two or more modes at different wavelengths all have the same group velocity
  • Figures 37 and 38 show measured output spectrum and simulated group index (respectively) from an example of two different intermodal Raman scattering processes taking place in a fiber;
  • Figures 39 - 43 present several specific configurations of the general arrangement of Figure 1;
  • Figure 44 shows a simulation result when a pre-chirp is such that a pulse initially compresses temporally as it propagates through a fiber
  • Figure 45 confirms scalability of the processes with respect to pulse repetition rate.
  • ultrafast, energetic near-infrared or mid-infrared sources may be of interest in applications such as (1) trace gas sensing for homeland security as well as environment monitoring applications; (2) accelerator applications such as high harmonic generation, proton and ion beam generation via gas jets, and advanced concepts for dielectric laser accelerators; and (3) machining applications such as heart stent manufacturing.
  • ultrashort typically denotes pulses with temporal widths of 300 femtoseconds or less
  • energetic pulses usually have energies greater than 1 nano- joule (nJ) and up to a 1 micro joule ( ⁇ ).
  • peak powers in these pulses approach, and even exceed, megawatt (MW) levels.
  • high repetition-rate usually signifies sources that emit pulses at the rates of 100 kHz or more.
  • One approach of obtaining a fiber laser at a desired wavelength is to start with a conventional doped fiber lasers as a pump laser and then convert the light to a desired wavelength via a nonlinear process in the fiber.
  • a conventional doped fiber lasers as a pump laser
  • convert the light to a desired wavelength via a nonlinear process in the fiber.
  • an optical pulse traveling through a medium will experience dispersion, causing the pulse to temporally broaden, meaning that different frequencies propagates at different velocities.
  • a soliton a special pulse solution referred to as a soliton. This is a stable pulse solution that remains transform limited (i.e., it maintains its shape and does not spread in time) as it propagates through the fiber.
  • a soliton is a pulse whose product between its temporal and spectral widths remains constant during propagation.
  • a soliton arises as a balance between the linear effect of anomalous dispersion and the nonlinear effect of self- phase modulation.
  • the effect of Raman scattering in the fiber leads to a continuous shift of the center wavelength of the pulse towards longer wavelengths, as phonon interaction causes a continuous transfer of energy from the short wavelengths of the pulse to the longer wavelengths.
  • a fiber laser generating light at 1300 nm may be realized by forming a soliton at lOxx nm, using an ytterbium fiber laser as a pump at an input end of the fiber, and obtaining an ultrafast pulse at 1300 nm at the output of the fiber by controlling either or both the length of propagation in the fiber or the input pulse energy.
  • This process is known as soliton self-frequency shifting (SSFS).
  • SSFS soliton self-frequency shifting
  • the soliton shifts to longer wavelengths it simultaneously transfers energy via a process of Cherenkov radiation typically to a wavelength shorter than the pump wavelength.
  • nonlinear processes with an ultrafast pump input in a fiber can be used to generate light at both longer and shorter wavelengths relative to the pump wavelength.
  • Light at longer wavelengths remains a soliton and thus transform limited, while a Cherenkov radiation mediated pulse disperses or spreads in time, so applications using this part of the generated spectrum may require external pulse compression using, for example, dispersive
  • Figure 1 shows a laser system in high-level schematic form. It includes a source subsystem 10, a higher-order mode (HOM) fiber 12, and an output subsystem 14.
  • the source subsystem 10 establishes an optical signal within the fiber 12, and the signal undergoes conversion in a manner described herein to generate a converted optical signal within the fiber 12.
  • This input optical signal is in some cases referred to as a "pump" signal, generated by a pump source of the source subsystem 10.
  • the converted optical signal may itself exit the fiber 12 as an output optical signal, or it may undergo other process(es) within the fiber 12 to generate the ultimate output optical signal, which is provided to the output subsystem 14 for further optical conditioning and use.
  • the output subsystem 14 in many cases includes components for extracting or otherwise processing one or more specific higher-order modes.
  • Soliton generation and propagation require that the utilized mode of fiber 12 exhibit anomalous dispersion (i.e., dispersion D > 0).
  • dispersion D > 0 anomalous dispersion
  • typical single moded silica glass fibers may not be used at wavelengths below 1300 nm, where they typically exhibit normal dispersion D ⁇ 0.
  • the energy of the soliton is proportional to both the dispersion and the effective area of the optical fiber mode, and also the spectral bandwidth of the soliton:
  • Anomalous dispersion at ⁇ 1 um can be achieved using high-confinement-geometry quasi- single-moded fibers, such as photonic crystal fibers (PCF).
  • PCF photonic crystal fibers
  • wavelength-shifted solitons pulses at 1300 nm to less than 1 nJ.
  • PCFs are inoperable at high powers, because the in-fiber intensities would be too high and cause pulse break up, material damage, etc.
  • Other SSFS-based approaches have employed special waveguide designs that yield anomalous dispersion using the LP 0 2 mode in a fiber, over a very narrow range.
  • pulse energies in these approaches have not exceeded a few nJ, because the aforementioned dispersion- vs-mode-area trade-off is only slightly relaxed.
  • This description presents a solution to this longstanding problem based on using SSFS in a class of fibers in which mode-areas can be orders of magnitude larger than in PCF, while still achieving high anomalous dispersion values for a variety of wavelength ranges.
  • One key aspect of this approach is the realization that higher order modes (HOMs) have dispersion zeros that shift to shorter wavelengths as mode order is increased.
  • HOMs higher order modes
  • the size of the waveguide which controls mode area and hence power-handling capability, may be designed independent of where the dispersion-zero lies.
  • the HOM fiber 12 may belong to either of two broad classes supporting
  • a first type are modes that resemble free-space Bessel beams, also called LP 0; m modes, where m stands for radial order. Such modes are stable to bend perturbations in a step index fiber, and hence very scalable in mode area. Their mode- dependent dispersion property has been used to realize nanosecond lasers using four-wave mixing, for example.
  • the second type are modes that carry orbital angular momentum (OAM), which may be stable in fibers that are even km long. Such modes may have multiple radial orders, though their main characteristic is the existence of helical phase that, for a beam with OAM of order J, represents L phase wraps around the beam. In analogy with the LP 0; m modes in a fiber, the dispersion-zero of these OAM modes also shifts to lower wavelengths as mode order L is increased.
  • Figure 2 provides a general schematic of a refractive index profile of a HOM fiber supporting LP 0; m modes. While a single index step is illustrated, in general multiple index steps may be used. Important fiber design parameters are the size of the index step and the diameter of the HOM guidance region, referred to as "guidance diameter" or GD.
  • the guidance region may be defined by adding dopants, e.g. adding germanium to increase the refractive index, or fluorine to decrease the index with respect to silica.
  • the guidance region could also be defined in multiple other ways as described further below.
  • the GD diameter of the guidance region could be anywhere between tens of microns up to several hundreds of microns, leading to effective areas of the modes of over 6000 ⁇ 2 .
  • Figure 3 shows an alternative fiber with a double cladding structure (two index steps) and thus multiple guidance regions.
  • the HOM fiber has a single moded core 20 up to 7 ⁇ of radius, and a HOM guidance region 22 with GD of about 50 ⁇ .
  • the inset shows the fiber face, with the different shades indicating the different index regions of the fiber, e.g., core and cladding. Characteristics of operation of this type of fiber are included in the description below.
  • the single moded core can be made UV sensitive allowing for inscriptions of long period gratings (LPGs), which will be discussed further below.
  • LPGs long period gratings
  • Figures 4 A and 4B illustrate another fiber having a guidance region diameter of 84 ⁇ and refractive index profile as shown. Other description below may be based on this fiber, as well as on a fiber having a GD of 105 ⁇ .
  • the guidance region is defined using dopants in silica glass.
  • Figures 5 A and 5B illustrate an example HOM fiber in which the guidance region is defined by low index polymer, providing for a higher index step.
  • Figure 5B shows a propagated LP 0; i5 mode in this fiber.
  • Figure 6 illustrates an alternative in which the guidance region is defined by a ring of airholes 30.
  • the fiber may be all silica except for the ring of air holes 30 that continue along the length of the fiber.
  • the guidance region is defined as the region 32 inside the ring of airholes. As the refractive index of air is 1, this leads to a large index step, which relates to the number of modes guided in the fiber (higher index step leads to more modes).
  • Figure 7 shows an example fiber using an alternative material (not silica), which might facilitate guidance in other spectral regions.
  • a mid-IR fiber is envisaged with chalcogenide glasses - As 2 Se 3 (Arsenic triselenide, refractive index of 2.8) in the core and AS2S3 in the cladding (Arsenic trisulfide, refractive index of 2.4), and with a core diameter of ⁇ 30 ⁇ .
  • This material composition provides transparency in the 1 ⁇ to 10 ⁇ wavelength regime.
  • Figure 7 also shows simulated intensity profiles of selected guided modes in this fiber - the LP 0; 4, LPo,8, and LP 0;11 modes.
  • Figures 8 A and 8B show an example of a fiber that can be used for guiding
  • OAM modes These fibers typically have a guidance region defined by a high index ring. To ensure stable OAM mode propagation, a large index step is required at either the outer or inner boundary of this ring.
  • a central air region 40 is surrounded by a high-index doped region 42, which is surrounded by a pure silica region 44.
  • HOMs in a fiber enable dispersion and mode-area to be independently tailored. And as shown from eq. (1) above, obtaining high energy soliton pulses requires high dispersion and large effective area.
  • Figure 9A shows plots of dispersion and effective area (respectively) versus wavelengths for a number of LP 0; m modes, based on the refractive index profile shown in Figure 3.
  • ZDW zero dispersion wavelength
  • Figure 9B provides an example of a spatial intensity distribution of the LP 0; 9 mode. The distribution has a single center spot and eight surrounding concentric rings.
  • Figures 10A and 10B illustrate that the dispersion curve of a single mode, in this example the LP 0; io mode, can be tailored by fiber design. These examples use the fiber profile of Figure 2.
  • Figure 10A shows guidance diameter being altered while keeping the index step fixed, while Figure 10B illustrates the index step being altered for fixed guidance diameter.
  • the exact shape of the dispersion curve will impact the soliton dynamics, so this ability to tailor the dispersion curve can be used to obtain specific desired performance. Using a combination of these two parameters (GD and index step) and furthermore by mode selection it is possible to obtain almost any desired dispersion profile.
  • Figures 11 A and 1 IB illustrate dispersion versus wavelength and effective area versus wavelength for selected modes in a fiber having a guidance region of 84 ⁇ . These characteristics are based on the fiber profile of Figures 4A and 4B. It may be noticed that the dispersion curves are similar to those shown in Figures 9A and 9B for a fiber in which the GD is 50 ⁇ , however, the effective areas are over 1100 ⁇ 2 , as opposed to 600 ⁇ 2 . Thus, by increasing the guidance diameter it is possible to increase the effective area of the supported modes in the fiber, while still obtaining a similar dispersion curves. In additional examples below, the effective area of the modes may exceed 1600 ⁇ 2 . Stable propagation of LP 0; m modes with areas over 6000 ⁇ 2 have been demonstrated. As seen from eq. (1), increasing area is one way to scale the soliton energy.
  • Figure 12 is a plot of dispersion versus mode order at a wavelength of 1050 nm for two different fiber types. As seen from eq. (1), another way to increase the soliton energy is by increasing the dispersion. Dispersion increases with mode order, and fibers having higher index steps generally support higher order modes. Thus, another way of scaling soliton pulse energy in HOM fibers is to increase the index step. Figure 12 illustrates dispersion characteristics for two different fibers.
  • the longer curve is for a low-index polymer fiber such as that of Figure 5, showing that dispersion greater than 200 ps/nm-km can be achieved for mode orders higher than about LP 0; 3o- This is contrasted with an analogous fiber having an index step only one fourth the magnitude (obtainable using fluorine dopants in all-glass fiber), where dispersion is limited to about 70 ps/nm-km for a mode order of LP 0; 2o- Figure 13 illustrates dispersion curves for a fiber such as that of Figure 6 in which a large index step is obtained by surrounding the guidance region with a ring of low index air holes. It can be seen that many modes are guided and that very high dispersion values can be obtained.
  • Figures 14 A and 14B show dispersion and effective area for the fiber of Figure 7, which operates in the mid-IR wavelength regime.
  • the ZDW is about 4 ⁇ (as seen from the bottom-most curve on the left plot).
  • anomalous dispersion and thus soliton operation
  • the ZDW is at about -2.2 ⁇ .
  • the concept is similar to that discussed above for silica fibers, except that for silica the ZDW is on the order of 1300 nm.
  • the plot at right shows effective area versus wavelength for the LP 0;11 mode, showing that the effective area is above 200 ⁇ 2 for this wavelength regime.
  • HOMs As mentioned above with reference to Figure 8, another class of HOMs are the OAM modes.
  • the intensity distributions of these modes consist of one ring or several concentric rings but without any center spot.
  • Their main characteristic is the existence of helical phase that, for a beam with OAM of order J, represents L phase wraps around the beam.
  • Figure 15 presents a simulation of the dispersive characteristics of OAM modes versus wavelength for a fiber such as that of Figure 8. Notice that the dispersion increases with increasing mode order L (and thus ZDW decreases). The insets show the spiral patterns of the modes, obtained by interfering a given OAM mode and a Gaussian beam. More spiral arms indicate higher J.
  • Figure 17 shows a system arrangement having a 1030-nm pump laser 50 is used that emits 370 fs pulses at a 120 kHz rep. rate.
  • a spatial light modulator (SLM) 52 encodes the spatial phase information on the Gaussian output beam of the laser 50 and thereby controls which specific mode is launched in the HOM fiber 54.
  • Figure 17 shows a general setup; alternative arrangements are given below. It should be noted that the laser 50 and SLM 52 can be viewed as one example of the source subsystem 10 of Figure 1.
  • FIG 18A shows more specific aspects of a setup that exhibits SSFS in LP 0; m mode.
  • the spatial light modulator (SLM) excites the LP 0; 9 mode in a 45-cm long HOM fiber, which is a custom double cladding fiber whose guidance diameter (GD) is 53 um with an index step of 0.02.
  • the refractive index profile for this fiber is shown in Figure 3.
  • Figures 19A - 19C show various values of interest.
  • Figure 19A shows the output spectra for progressively increasing launched pump pulse energies (spectra vertically offset for clarity). Initially, at the onset of nonlinearities, the soliton is still merged with the continuum, and emission from Cherenkov radiation starts to build up at -800 nm (as theoretically predicted). As the launched power is increased, a clear shifted fundamental soliton is evident and spectrally separated from the rest of nonlinear products, thus allowing for spectral filtering and use as a standalone ultrafast source.
  • spectrally separated emission first appears at 1207 nm, and thereafter, the soliton shifts continuously to 1317 nm as the pump energy is increased (the dashed black line tracks the spectral peak position of the shifted soliton).
  • the yellow trace (second from above) a second first order soliton is observed at approximately 1100 nm, which will also be in the LP 0; 9 mode.
  • the output pulse energy of the fundamental solitons are measured using a power meter along with spectral filters to isolate the soliton from the spectrum.
  • Figure 19B shows an example of the above.
  • the lower trace is the filtered spectrum using a 1250 nm longpass filter.
  • the inset is an image of the filtered soliton which clearly shows that the output is a pure LP 0; 9 (same as the pump mode).
  • the measured energy of this shifted soliton at different wavelengths is denoted next to the spectral traces in Figure 19A - a 30-nJ pulse at 1317 nm is obtained, the farthest shift in this experiment.
  • Figure 19C shows measured autocorrelation (full width at half maximum - FWFDVI) width is 82.7 fs, which corresponds to a pulse, with secant-hyperbolic distribution in time (sech 2 pulse) of 53.6 fs duration.
  • the measured spectral bandwidth is 50 nm (FWHM), which represents a transform limited pulse width of 39 fs.
  • FWHM nm
  • the measured bandwidth product is 0.432 as opposed to the transform limited 0.315 for a sech 2 -pulse. This additional broadening may be due to optical components between the HOM fiber and the autocorrelator.
  • the above demonstrates soliton shifting in a 660- ⁇ 2 A e ff LP 0; 9 mode of a multimode fiber.
  • Wavelength shifts of almost 300 nm are obtained, so that the technique can access the technologically important 1300-nm spectral range.
  • Peak powers of -0.56 MW and pulse energies of 30 nJ are obtained for an ultrashort pulse (53.6 fs) directly emitted from a fiber laser with no external pulse compression.
  • the output at 1300 nm is a highly spatially coherent beam, which allows conversion back to a Gaussian-shaped beam with a simple axicon device.
  • the technique is scalable to higher pulse energies as well as extendable to other spectral ranges, such as the mid-IR.
  • Figure 20 illustrates SSFS in a low-index polymer coated fiber such as that of Figure 5, using a setup similar to that illustrated in Figure 17.
  • the LP 0; i7 mode is launched.
  • the obtained spectrum is shown in Figure 20, and the inset shows the mode image obtained by spectrally filtering out the right-most peak at -1190 nm.
  • the mode image in this case does not consist of concentric rings, but rather has a "bowtie" shape. This is due to a change in the eigenbasis for this fiber caused by vectorial effects due to the large index step at the boundaries. This is however still a single spatially coherent mode.
  • Figures 21 A and 2 IB show soliton shifting spectra and mode image respectively in air-cladding fiber such as that of Figure 6.
  • the LP 0; io mode is excited.
  • Figure 2 IB shows the mode image of the spectrally filtering peak at 1090 nm in the blue trace, which is also the LP 0; io mode, same as the pump mode.
  • Figure 22 shows results of soliton shifting in an OAM mode in the air core fiber of Figure 8, whose dispersion curves are shown in Figure 16.
  • Figure 22 shows obtained spectra for both low (narrow trace) and high (wider trace) pump power.
  • Figure 23 illustrates another aspect of the system configuration, namely how fiber length can be used to control the output energy of the soliton at a certain wavelength. These measurements are based on the same fiber and system as presented in Figure 19. In this experiment a mode was launched in a fiber sample and for different launched pump powers the soliton achieved different wavelength shifts, the soliton energy was measured at these different wavelengths. This was repeated for different fiber lengths.
  • Figure 23 illustrates that, for a given desired wavelength output (e.g., 1300 nm), controlling the fiber length enables controlling the soliton energy - specifically, the pulse energy obtained at a given wavelength increases with decreasing fiber length.
  • the required launched pump power increases as the fiber length is decreased to ensure the desired shift (e.g., out to 1300 nm).
  • Figure 24 shows a desired dispersion profile for the LP 0; 9 mode of a HOM fiber in which temporal pulse narrowing may be achieved.
  • the energy of the soliton is proportional to the dispersion, the effective area, and the spectral bandwidth of the soliton. In this section it is described how this relationship can lead to temporal pulse compression of the soliton.
  • the soliton As the soliton shifts towards longer wavelength beyond 1210 nm the dispersion of the modes starts to decrease. Thus in order for the soliton to fulfil eq. (1) as it continues to propagate through the fiber, the soliton must reshape itself, because its energy remains nearly constant. This reshaping leads to an increase in spectral bandwidth to counteract the decreasing dispersion, meaning that the pulse reshapes itself into one of narrower duration.
  • Figure 25 shows the above effect, i.e., that soliton spectral bandwidth increases with increasing wavelength shift (obtained by increasing the pump power).
  • the spectral width of the pulse increased from about 32 nm to 47 nm, meaning the supported pulse width decreases from 50 fs to 40 fs. Note that similar results can be obtained by holding pump power constant and modifying the length of the fiber.
  • This section describes a separate phenomenon in multimoded fibers, which is referred to as intermodal Raman scattering for reasons that will be apparent.
  • This process can provide even wider wavelength tunability than the above SSFS process, further enhancing the prospects of a platform for building tunable-wavelength ultrafast lasers.
  • the process is related to interpulse and intermodal discrete energy transfer between two pulses traveling with the same group velocity through a fiber, as is described in more detail below.
  • the process of Raman scattering is the interaction between two photons and a phonon (a molecular vibration).
  • a phonon a molecular vibration
  • a high energy (low wavelength) photon interacts with the material and creates a lower energy (higher wavelength) photon and a phonon.
  • Figure 26 shows the Raman gain coefficient for silica as a function of wavelength.
  • the pump photon is at 1250 nm, but as this is self-phase-matched process (meaning that the two photons - incident and scattered, and the phonon, automatically adjust their relative phases to make this process efficient), the pump could be at any wavelength.
  • An important aspect is that, in silica fibers, the peak of the Raman gain curve is always at 13 THz lower in frequency than the pump. So in this example, if the pump is at 1250 nm the gain peak will be at 1321 nm as seen in Figure 26 (note that lower frequency corresponds to longer wavelength).
  • the efficiency of this Raman coupling process is proportional to the intensity overlap integral f j i between the two interacting optical modes, which is given in eq. (2) below:
  • the horizontal line is used to indicate that numerous modes at different wavelengths have this exact same group. This means that, for example, the LP 0;8 mode at 1300 nm and the LP 1;7 mode at 1390 nm travel through the fiber with the same group velocity. Hence, these two modes at different wavelengths do not temporally walk-off from each other.
  • the vertical line at 1380 nm is exactly 13 THz away from the vertical line at 1300 nm for the LPo ;8 mode.
  • the vertical line at 1480 nm is 13 THz away from the vertical line at 1390 nm for the LP 1;7 mode, and so on. This illustrates that the spacing in frequency between these group index matched modes indicated by the horizontal line all are separated by approximately 13 THz, and thus these modes can transfer energy to each other via Raman scattering as this coincides with the peak of the Raman gain coefficient.
  • Figure 28 shows a plot of effective Raman area [inverse of eq. (2)] versus cascade order for the process of Figure 27.
  • Intermodal Raman scattering requires that the overlap integral in eq. (2) be non-zero.
  • the effective Raman area is the inverse of the overlap integral.
  • the horizontal axis represents one or multiple instances of this intermodal process occurring, since once a new wavelength has been created by this process, light with sufficient energy at this wavelength may act as a pump for a subsequent intermodal interaction of a similar kind, to an even longer wavelength.
  • Each instance of this intermodal Raman scattering occurrence is henceforth identified as a cascade number, to signify that this process may be cascaded in nature.
  • the number one cascade order is the effective Raman area between the LP 0;8 mode at 1300 nm and the LP 1;7 mode at 1390 nm
  • the number two cascade order is the effective Raman area between the LP 1;7 mode at 1390 nm and the LP 0; 7 mode at 1480 nm, and so on.
  • the upper trace on the plot is furthermore weighted with the strength of the Raman gain coefficient at these wavelength separations. What this indicates is that the process where the effective Raman area weighted by the Raman gain coefficient is smallest is the process that will dominate. Hence, for this fiber, it is expected that light will experience discrete Raman cascades up to the fourth order. Beyond that, the weighed area becomes too large for the process to be efficient. This points to a way, by means of fiber design, to control the number of cascades or wavelength shifts that occur within a fiber.
  • Figure 29 shows a comparison between the strength of different cascade processes.
  • the y-axis is the overlap integral multiplied by the Raman gain coefficient at the subject wavelength separation - thus, this axis plots a parameter that is approximately the reciprocal of the parameter plotted in the y-axis of Fig. 28.
  • the process with the largest value is the most efficient and hence the process most likely to occur in the fiber.
  • the process '+ indicates that the coupling goes to nearest group index matched mode, so in this case from LP 0; 8 -> LP 1,7 -> LPo,7 -> LP 1;6 -> LP 0; 6 and so on.
  • the process '+2' indicates that a nearest group index matched mode is skipped, so in this case from LP 0; 8 -> LPo,7 -> LPo,6 and so on.
  • the process '+3' indicates that two nearest group index matched modes are skipped, so in this case from LP 0; 8 -> LPi,6 -> LPo,5 and so on.
  • Figures 30 and 31 show similar data for a fiber having a GD of 105 ⁇ , while the results of Figures 28-29 are for a fiber having a GD of 50 ⁇ .
  • the group index curves are more closely spaced.
  • the LP 0; i8 mode at 1190 nm has the same group index as both the LP 1;17 mode at 1230 nm and the LP 0; i7 mode at 1270 nm.
  • the LP 0; i8 mode is more likely to couple to.
  • the '+2' process is more likely as that trace is larger in value. This means that this process is expected to go only between LP 0; m modes, in this case LP 0; i8 -> LP 0; i7 -> LP 0; i6.
  • Figure 32 shows an output spectrum for an experiment in which the pump pulse is launched in the LP 0; i8 mode in a fiber with a guidance region of 105 ⁇ .
  • the pump pulse is launched in the LP 0; i8 mode in a fiber with a guidance region of 105 ⁇ .
  • two solitons in the LP 0; i8 mode are formed at -1080 nm and ⁇ 1187 nm, which is similar to the behavior observed in Figure 19 for traditional SSFS, although with the additional feature of being at much higher energies than was previously possible.
  • two additional spectral features appear at 1271 and 1352 nm, these are in the LP 0; i7 and LP 0; i6 modes, respectively.
  • Figure 33 illustrates a simulation showing that the LP 0; i8 mode at 1187 nm, the LP 0; i7 mode at 1271 nm, and the LP 0; i6 mode at 1352 nm all have the same group index, meaning all these wavelengths travel though the fiber at the same group velocity. Furthermore, these three wavelengths are spaced by approximately 13 THz, which is at the peak of the Raman gain.
  • the pump in one mode creates a soliton that shifts to a longer wavelength via traditional SSFS. This shifted soliton serves as a second pump signal for generating one or more signals in other modes via intermodal Raman scattering.
  • the group velocity of the shifted- soliton mode matches (i.e. is close or equal to) that of another mode, spectrally separated by an amount roughly equal to that where the peak Raman gain occurs (typically 13 THz in silica fibers).
  • the intermodal Raman scattering process not only allows creating ultrashort energetic pulses at new colors, it vastly enhances the tuning range since shifts as far away as multiple Raman shifts (i.e.
  • any given embodiment may utilize one or more than one stages in the cascade to achieve a desired final wavelength. In the example shown above the cascade occurred between LP 0; m modes that are all azimuthally invariant. In general, this is not required.
  • Figure 34 shows an experiment in which the LP 0; 8 mode is launched in a fiber with a guidance diameter of about 50 ⁇ . The following spectral features are marked:
  • 'a' is Cherenkov radiation and is in the LP 0; 8 mode
  • Figures 35 A - 35E show mode images of the spectral features 'c' through 'f of Figure
  • Figure 36 presents plots of simulated group index showing that the LP 0; 8 mode at -1330 nm, the LP 1 7 mode at -1420 nm and the LP 0;7 mode at -1510 nm all have the same group velocity, and for this fiber these modes are separated by approximately 13 THz (peak of the Raman gain). As the intensity overlaps between these modes are non-zero, this is the cascaded process observed.
  • Figures 37 and 38 show measured output spectrum and simulated group index (respectively) from an example of two different cascaded processes taking place in the same fiber.
  • the launched mode in this case is the LP 0; i7 mode.
  • the peaks marked as 'b' and 'c' have the same group index (the lower horizontal dashed line in Figure 38), whereas the peaks marked by 'd', 'e', 'f , 'g', and 'h' all have the same group index (the upper horizontal dashed line in Figure 38).
  • Our invention paves the path for an all-fiber monolithic architecture requiring no manual tuning or alignment, resistance to thermal, mechanical and other environmental perturbations, low weight, small footprint and potentially lower cost.
  • Figures 39 - 43 present several specific configurations of the general arrangement of Figure 1 in greater detail along with their operation principles.
  • multimode fibers may support large numbers of modes (e.g., up to thousands), it is important to have the ability to excite just one or a few of the desired HOMs. In many cases the goal may be to excite a HOM in a fiber starting with a nearly Gaussian output from a pump laser.
  • the source-side mode converter is a free space element, which may be realized by a spatial light modulator (SLM), a Micro- Electro-Mechanical System (MEMS), a phase plate (e.g. binary phase plate), or axicon lens for example.
  • SLM spatial light modulator
  • MEMS Micro- Electro-Mechanical System
  • phase plate e.g. binary phase plate
  • axicon lens for example.
  • the nonlinear interaction takes place and light is converted to a desired wavelength.
  • an additional mode conversion takes place using any of the aforementioned techniques, and the output could at the end for example be used for an imaging microscope, such as multiphoton microscope.
  • the mode conversion after the fiber could also take care of aberration corrections to improve the focus of light into the sample.
  • Figure 40 shows an arrangement having No output conversion: In some cases, depending of the application of the laser it may also be desirable to not re-convert the output mode and instead use the HOMs directly out of the laser, since Bessel beams
  • Figure 41 shows Fiber facet mode excitation:
  • the source-side mode converter is fabricated directly on the end facet of the fiber.
  • Examples include a phase plate (e.g.
  • Figure 42 shows Long period grating (LPG) mode excitation:
  • the mode conversion takes place in the fiber using a long period grating (LPG). This could either be a UV-induced grating or an acoustic grating.
  • LPG long period grating
  • This could either be a UV-induced grating or an acoustic grating.
  • the nonlinear conversion takes place and light is converted to a desired wavelength.
  • an grating LPG
  • Figure 43 shows Long period grating (LPG) mode excitation and dispersion control (DC):
  • LPG long period grating
  • DC preceding dispersion control
  • the nonlinear interaction takes place and light is converted to a desired wavelength.
  • an additional mode conversion could takes place using either SLM, MEMS, axicon, phaseplates, the latter two either as free space elements or fabricated directly on the fiber facet.
  • Dispersion control may be desirable if a pre-chirped approach described below is
  • LPG long period fiber gratings
  • BPP binary phase plates
  • axicons Various approaches to mode conversion at the input or output, as depicted in Figures 39-43, include long period fiber gratings (LPG), acousto-optic fiber gratings, binary phase plates (BPP) or axicons.
  • LPGs long period fiber gratings
  • BPP binary phase plates
  • axicons The operation of LPGs for the purposes of mode conversion is described in S. Ramachandran, "Dispersion-tailored few-mode fibers: a versatile platform or in-fiber photonic devices, " J. Lightwave Tech., vol. 23, p. 3426, 2005, which is hereby incorporated by reference in its entirety.
  • BPPs and axicons for the purposes of mode conversion is described in J. Demas, L. Rishoj and S.
  • Figure 44 shows a simulation when the pre-chirp is such that the pulse compresses temporally as it propagates through the fiber in the anomalous dispersion mode.
  • an unchirped pulse is launched into this fiber, it splits into two solitons, and at the output of the fiber these have shifted to ⁇ 1100 and -1700 nm, respectively.
  • the pulse compresses in time while the spectral bandwidth remains nearly unchanged. Eventually, after about 32m of propagation, the pulse has become so compressed that the peak power is high enough that the nonlinearity becomes a factor, producing a soliton.
  • the SSFS process only happens over a shorter effective length, leading to a smaller wavelength shift to about 1350 nm.
  • the degree of pre-chirp allows for continuous tuning from the 1050 nm pump wavelength all the way to 1700 nm. Even though dispersion and nonlinear effects to some extent always occur simultaneously, the
  • aforementioned step-wise aspect of this process namely, that a chirped pulse first undergoes temporal compression and then experiences nonlinear wavelength tuning (either via SSFS or cascade) - enables understanding how chirp may be used a wavelength tunability parameter. This is especially interesting because amount of dispersive chirp can easily be electronically (or robustly mechanically) controlled by a variety of dispersion control devices for ultrashort pulses.
  • Figure 45 illustrates dynamics related to pulse repetition rate.
  • the repetition rate of the laser is 120 kHz.
  • the repetition rate of the laser is 120 kHz.
  • Figure 45 illustrates this for a soliton shifting experiment similar to the one described with reference to Figure 19. Note that nearly the same wavelength shift is obtained using the same fiber sample regardless of whether the repetition rate of the pump laser is 120 kHz or 1 MHz, provided that the pulse energies of the individual pulses in the pulse trains are kept constant.
  • the minor discrepancy seen in the shifted wavelength at the two repetition rates is related to a slight change in the pulse shape when changing the pulse repetition rate on the pump laser.
  • the processes described herein for producing wavelength tunable ultrashort pulses either via SSFS (in an energy scalable fashion) or via intermodal Raman scattering (at low as well as high pulse energies) is independent of the repetition rate.
  • these processes may be used for any repetition rate source that the pump laser possesses, or alternatively the system can be designed for any repetition rate needed by an application. While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

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Abstract

L'invention concerne une fibre de mode d'ordre supérieur (HOM) d'un laser à fibre ayant un saut d'indice et un diamètre de guidage (GD) définissant des caractéristiques de dispersion dépendant de la longueur d'onde et des surfaces utiles pour des HOM correspondants de propagation de signal optique. Un HOM présente une dispersion anormale et une surface utile définissant une première longueur d'onde et une première puissance d'un signal optique d'impulsion en vue d'une conversion en une seconde longueur d'onde et en une seconde puissance par auto-décalage en fréquence du soliton (SSFS). En commandant le saut d'indice et le GD, la dispersion et la surface utile d'un HOM sont ajustées pour amener la seconde longueur d'onde dans une plage souhaitée, ce qui permet de mettre en œuvre des applications nécessitant des longueurs d'onde de laser à fibre non classiques. Les HOM peuvent partager un indice de groupe et une vitesse de groupe prédéfinis à des longueurs d'onde établies par un pic de gain de Raman pour effectuer une conversion de longueur d'onde par diffusion Raman interimpulsions et intermodale, qui peut se produire en cascade pour produire des lasers multicolores ayant des longueurs d'onde, des énergies d'impulsion et des largeurs d'impulsion souhaitées.
PCT/US2017/017645 2016-02-12 2017-02-13 Laser à fibre à impulsions ultracourtes employant la diffusion raman dans des fibres de mode d'ordre supérieur WO2017192196A2 (fr)

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